Electromagnetic spectrum

Last updated: December 15, 2009.

You might think the world is essentially what you can see in front of you, but think for a moment and you'll realize this isn't true. When you close your eyes, the world doesn't cease to exist just because there's no light to see by. If you were a rattlesnack or an owl, you could see perfectly well by night. Thinking more laterally, what if you were a radar set mounted on an airplane? Then you could help pilots see in darkness or bad weather by detecting reflected radio waves. And if you were a camera sensitive to X rays, you could even see through bodies or buildings! The light we can see is only one part of all the electrical and magnetic energy buzzing around our world. Radio waves, X rays, gamma rays, and microwaves work in a very similar way. All together, this energy is called the electromagnetic spectrum. Let's take a closer look at what that means!

Photo: Water drops bend (or refract) short wavelength blue light more than long-wavelength red light, which is why rainbows arc across the sky when sunlight streams through rain.

What is electromagnetic radiation?

Light waves and other types of energy that radiate (travel out) from where they're produced are called electromagnetic radiation. Together, they make up what's known as the electromagnetic spectrum. Our eyes can see only a limited part of the electromagnetic spectrum—the colorful rainbow we see on sunny-rainy days, which is only a tiny part of all the electromagnetic radiation that zaps through our world. We call the energy we can see visible light (we discuss it in detail in our main article on light) and, like radio waves, microwaves, and all the rest, it's made up of electromagnetic waves. These are up-and-down, wave-shaped patterns of electricity and magnetism that race along at the speed of light (300,000 km per second or 186,000 miles per second, which is fast enough to go 400 times round the world in a minute!). The light we can see stretches in a spectrum from red (the lowest frequency and longest wavelength of light our eyes can register) through orange, yellow, green, blue, and indigo to violet (the highest frequency and shortest wavelength we can see).

What are the other kinds of electromagnetic radiation that objects give off? Here are a few of them, ranged in order from the longest wavelength to the shortest:

• Radio waves: If our eyes could see radio waves, we could (in theory) watch TV programmes just by staring at the sky!
• Microwaves: Obviously used for cooking in microwave ovens, but also for transmitting information in radar equipment. Microwaves are like short-wavelength radio waves.
• Infrared: Just beyond the reddest light we can see, with a slightly shorter frequency, there's a kind of invisible "hot light" called infrared. Although we can't see it, we can feel it warming our skin when it hits our face—it's what we think of as radiated heat. If, like rattlesnakes, we could see infrared radiation, it would be a bit like having night-vision lenses built into our heads.
• Ultraviolet: This is a kind of blue-ish light just beyond the highest-frequency violet light our eyes can detect. The Sun transmits powerful ultraviolet radiation that we can't see: that's why you can get sunburned even when you're swimming in the sea or on cloudy days—and why sunscreen is so important.
• X rays: A very useful type of high-energy wave widely used in medicine and security. Find out more in our main article on X rays.
• Gamma rays: These are the most energetic and dangerous form of electromagnetic waves.

Photo: Ultraviolet lamps like this can be used to show up the "invisible" security inks that deter thieves. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).